The Harbor Freight guages are not the best, BUT, will do the job. I have 2 sets. One R12 the other R134a. Get the book. It will help with guage diagnosis and information. I do not know if the book has info on your car. But my book does.
Haynes Techbook, Automotive Heating and air Conditioning

1991 190#E 2.6

Low side guage 24-34 psi
High side guage 230-275 psi
5.00 Oz of oil
2.2 Lbs of freon.

Now this is for R12. If you convert your system, make sure you do it right, Oring, flush etc, The amounts for R134a freon will change.

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car